The present invention is related to a deep drawable sheet or strip of metal, preferably aluminum or an aluminum alloy having a surface treated for deep drawing and a method for the manufacture of such a sheet or strip.
A deep drawable sheet metal is known from German patent application DE-OS 30 08 679. Due to the roughing methods employed in its manufacture, the sheet has an irregular microroughness (Ra) of approximately 0.8 to 5 .mu.m with primarily slender peaks finely distributed over the surface of the metal. The irregularity refers to the height and diameters of the peaks in the distance and the outline of the depressions in the surface, respectively. In connection with the holding-down clamp employed to produce the sheet, the depressions are intended to be sealed off during deep drawing, so that the lubricant filling which is taken up remains enclosed (see column 3, line 60 of the '679 patent).
In order to achieve the above-described effect, the roll surface is roughened by sand blasting (in the method according to German patent application DE-OS 30 08 679) and an undesirable microsurface is produced through rerolling of the sheet or the strip.
In the case of the deep drawable sheet or strip according to DE-OS 30 08 679 roughened in the manner described above, the depressions for the lubricant are arranged staggered with respect to each other. However, under closer investigation it can be seen that the microdepressions are connected with each other, so that the lubricant filling that is taken up can flow off laterally. This can cause folds and/or tears at the sites which are exposed to the greater stresses of the drawing operation.
What is needed in the art is a method for deep drawing a metal sheet or strip of aluminum or of an aluminum alloy having a hydrodynamic pressure structure (in terms of the lubricant) in the compression gap between the tool surface and sheet roughness peaks in order to improve the slide properties in the drawing tool and to decrease the tensile force required by at least 30% compared to conventional surfaces. Products produced by deep drawing sheets or strips of aluminum using the method of the present invention include automobile parts, beverage cans and plates.
Therefore, it is an object of the present invention to provide a method for deep drawing a sheet or strip metal which overcomes the above-mentioned drawbacks in the prior art techniques.
This and other objects of the present invention will be apparent to those of ordinary skill in the art in light of the present description, accompanying claims and appended drawings.